Context Locals

Sooner or later you have some things you want to have in every single view or helper function or whatever. In PHP the way to go are global variables. However, that isn’t possible in WSGI applications without a major drawback: As soon as you operate on the global namespace your application isn’t thread-safe any longer.

The Python standard library has a concept called “thread locals” (or thread-local data). A thread local is a global object in which you can put stuff in and get back later in a thread-safe and thread-specific way. That means that whenever you set or get a value on a thread local object, the thread local object checks in which thread you are and retrieves the value corresponding to your thread (if one exists). So, you won’t accidentally get another thread’s data.

This approach, however, has a few disadvantages. For example, besides threads, there are other types of concurrency in Python. A very popular one is greenlets. Also, whether every request gets its own thread is not guaranteed in WSGI. It could be that a request is reusing a thread from a previous request, and hence data is left over in the thread local object.

Werkzeug provides its own implementation of local data storage called werkzeug.local. This approach provides a similar functionality to thread locals but also works with greenlets.

Here’s a simple example of how one could use werkzeug.local:

from werkzeug.local import Local, LocalManager

local = Local()
local_manager = LocalManager([local])

def application(environ, start_response):
    local.request = request = Request(environ)
    ...

application = local_manager.make_middleware(application)

This binds the request to local.request. Every other piece of code executed after this assignment in the same context can safely access local.request and will get the same request object. The make_middleware method on the local manager ensures that all references to the local objects are cleared up after the request.

The same context means the same greenlet (if you’re using greenlets) in the same thread and same process.

If a request object is not yet set on the local object and you try to access it, you will get an AttributeError. You can use getattr to avoid that:

def get_request():
    return getattr(local, 'request', None)

This will try to get the request or return None if the request is not (yet?) available.

Note that local objects cannot manage themselves, for that you need a local manager. You can pass a local manager multiple locals or add additionals later by appending them to manager.locals and every time the manager cleans up it will clean up all the data left in the locals for this context.